Cass County is part of the Fargo, ND–MN Metropolitan Statistical Area.
As of the census of 2000, there were 123,138 people, 51,315 households, and 29,814 families residing in the county. The population density was 70 people per square mile (27/km²). There were 53,790 housing units at an average density of 30 per square mile (12/km²). The racial makeup of the county was 95.10% White, 0.81% Black or African American, 1.08% Native American, 1.26% Asian, 0.03% Pacific Islander, 0.43% from other races, and 1.29% from two or more races. 1.23% of the population were Hispanic or Latino of any race. 34.1% were of German and 32.4% Norwegian ancestry according to Census 2000.
There were 51,315 households out of which 29.9% had children under the age of 18 living with them, 47.3% were married couples living together, 7.6% had a female householder with no husband present, and 41.9% were non-families. 31.2% of all households were made up of individuals and 7.6% had someone living alone who was 65 years of age or older. The average household size was 2.32 and the average family size was 2.98.
In the county the population was spread out with 23.4% under the age of 18, 16.0% from 18 to 24, 31.3% from 25 to 44, 19.6% from 45 to 64, and 9.7% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 100.3 males. For every 100 females age 18 and over, there were 99.5 males.
The median income for a household in the county was $38,147, and the median income for a family was $51,469. Males had a median income of $32,216 versus $22,300 for females. The per capita income for the county was $20,889. About 5.7% of families and 10.1% of the population were below the poverty line, including 9.1% of those under age 18 and 8.1% of those age 65 or over.